Translations:Rust installation special notes in China/4/en

Some recommended choices
RUSTUP_UPDATE_ROOT=https://mirrors.ustc.edu.cn/rust-static/rustup RUSTUP_DIST_SERVER=https://mirrors.ustc.edu.cn/rust-static [source.crates-io] replace-with = 'ustc'
 * USTC open source software mirror (https://mirrors.ustc.edu.cn/)